Coagulation device comprising an energy control

ABSTRACT

A device ( 10 ) for tissue coagulation, in particular for fusion, encompasses an electric source ( 18 ), which is connected or which can be connected to electrodes ( 12, 13 ) for influencing biological tissue ( 11 ) with current. A control unit ( 22 ) controls the source ( 18 ) during phases I and II of the tissue fusion. These phases I and II correspond to operating phases I, II and III of the device ( 10 ). During operating phase I, a monitoring unit ( 23 ) determines the energy E 1 , which is applied into the tissue ( 11 ). In the subsequent operating phases II and III, the control unit ( 22 ) controls the source ( 18 ) by means of the determined energy E 1 . Such a device turns out to be particularly reliable and to be robust in use.

RELATED APPLICATION(S)

This application claims the benefit of European Patent Application No.13169105.7 filed May 24, 2013, the contents of which are incorporatedherein by reference as if fully rewritten herein.

TECHNICAL FIELD

The invention relates to a device for tissue coagulation, in particularfor tissue fusion.

BACKGROUND

Various electrosurgical methods, the effect of which is based on acontrolled denaturation of biological tissue, are in use.

For example, EP 1 862 137 A1 discloses a coagulation device comprising agenerator, which feeds two electrodes, between which biological tissueis seized. During the coagulation, the tissue runs through a first phaseI, during which the tissue impedance decreases considerably, and asecond phase II, during which the tissue impedance increases again. Todetermine the tissue impedance, provision is made for a sensor circuit,which transmits a query signal, so as to determine the initial tissueimpedance and so as to subsequently define a certain trajectory for thedesired course of time of the tissue impedance. The query signal isformed by means of an electric pulse, by means of which a tissuecharacteristic is measured. The measured tissue characteristic can beenergy, power, impedance, current, voltage, electric phase angle,reflected power or temperature.

U.S. Pat. No. 8,216,223 B2 also deals with the coagulation of tissue.The tissue impedance is initially measured during an HF activation ofelectrodes. Over the course of time, the minimum of the impedance isestablished. Starting at this point, a reference value curve isgenerated for the desired impedance increase and a target value iscalculated for the impedance. Once the latter has been reached, the HFgenerator is turned off. The turn-off is followed by a cooling phase,the length of which is also provided by the reference value curve. Thefusion is concluded with the end of the cooling phase.

The thermofusion according to U.S. Pat. No. 8,034,049 B2 is alsocontrolled by means of the initial tissue impedance. In phase I of thethermofusion, the course of the impedance is measured in response tocurrent, which is kept constant, for example. The initial impedance, thedecrease of the impedance, the minimum of the impedance or the increaseof the impedance are derived from this. Other activation parameters aregenerated from this information.

EP 2 213 255 B1 describes the control of the energy in response to athermofusion. A state variable SV, which indicates the decrease orincrease of the impedance, is generated for this purpose. A referencevalue trajectory is provided for the impedance. The energy input iscontrolled such that the desired chronological course of the impedanceis approximated. For this purpose, the energy input is coupled orcountercoupled as a function of the state variables SV to the impedance.

EP 2 394 593 A1 describes the measuring of the impedance during thethermofusion. Provision is made to check, whether, after a certainminimum time has lapsed, a minimum impedance has been reached. As soonas this is the case, the activation is concluded.

U.S. Pat. No. 6,733,498 B2 discloses a method for thermofusion, in thecase of which the chronological course of the tissue impedance isdetermined during the application of HF voltage. The end of the firstphase and the duration of the second phase are defined accordingly bymeans of the course of the impedance.

U.S. Pat. No. 8,147,485 B2 also uses the monitoring of the tissueimpedance for regulating the thermofusion. An impedance trajectory iscalculated from the minimum of the tissue impedance and the impedanceincrease.

U.S. 2010/0179563 A1 and U.S. 2011/0160725 A1 also determine the tissueimpedance or the change thereof for controlling or regulating theelectrosurgical process.

The local state of tissue is characterized by the local specific tissueimpedance. Even though the determination of the impedance between twoelectrodes provides an indication for the state and thus for thetreatment progress of the tissue as a whole, the local specific tissueimpedance, however, is not determined. This can lead to incorrectconclusions in the case of inhomogeneous tissue.

SUMMARY

It is the task of the invention to create an alternative device fortissue coagulation.

The device according to the invention serves the purpose of tissuecoagulation and, if necessary, also tissue fusion. For this purpose, anelectric source is connected or can be connected to electrodes forinfluencing biological tissue with current. The electric source can be asource for direct current or alternating current, preferably HF current.Preferably, the source is embodied in a controllable manner, so as to beable to control the size of the output current and/or of the outputvoltage. For this purpose, said source is connected to a control unit.The latter includes a monitoring unit, which is connected to the source.In particular, the monitoring unit is connected to the output of thesource, to which the electrodes are connected as well. In thealternative, the monitoring unit can be connected to the electrodes. Themonitoring unit thus determines at least an electric variable, whichcharacterizes the energy, which was output from the source to theelectrodes and thus from the electrodes to the tissue during a firstoperating phase. The first operating phase corresponds to phase I of thetissue coagulation, during which the tissue resistance decreases andpasses through a minimum.

For example, the monitoring unit can determine the current power and canintegrate it during the first operating phase, so as to establish theoutput energy. It is advantageous in particular, if the monitoring unitdetermines the active power, which is output by the electrodes. By meansof integration, the active energy, which was converted thermally in thetissue, is established from this. The energy, which was input into thetissue in the first operating phase, is used to control the secondoperating phase. The latter corresponds to phase II of the tissuecoagulation, during which the tissue resistance increases and the tissuedries by boiling tissue fluid.

In the alternative, the apparent power, which, however, includes idlepower portions, can be determined. If said idle power portions are knownor constant, the apparent power and thus the total apparent energy,which was output, can also be used to control the second operatingphase.

The control unit controls the source in the second operating phase bymeans of the energy (active energy or apparent energy), which wasestablished during the first operating phase. It is ensured through thisthat the energy quantity applied in the second operating phase isadapted to the size of the tissue area, which is determined andinfluenced by the electrodes. The cells, which are open in the firstphase I, release tissue fluid. In the second phase II, said tissue fluidis evaporated by drying the tissue. By determining the energy, which wasapplied in the first operating phase, a parameter is available, by meansof which phase II can be controlled such that the entire tissue, whichwas influenced electrosurgically in phase I, is coagulated evenly.

It is advantageous, if the control unit operates the source in the firstoperating phase I by means of a regulated current. At the onset, it isthereby possible to provide a chronologically increasing current as wellas a constant current in the further course of the operating phase I.This results in a heat-up of the tissue and in a heat-up of theelectrodes. Thermal tissue denaturation results in a decrease of thetissue impedance, which can be between 2 Ohm and 40 Ohm, for example.Due to vapor formation and beginning drying of the tissue, the impedancecan increase again during operating phase I, until the end of phase I isrecognized. Various recognition criteria can be used for this purpose.For example, the relationship between voltage and current at the sourceand thus the tissue impedance can increase beyond a threshold value. Inthe alternative, it can be used as recognition criterion, if therelationship between voltage and current at the source, that is, thetissue impedance, passes through a minimum. As a further alternative, itcan be used as recognition criterion that the voltage at the sourceexceeds a threshold value. As a further alternative, it can be used asrecognition criterion that the current, which is to be kept constant bythe source, falls below a threshold value, because the currentregulating circuit formed by the control unit and the source leaves itscontrol range. This can take place, when the source has reached itsmaximum voltage or another voltage limit. In the alternative, the speedof the change of the tissue resistance (relationship between voltage andcurrent at the source) can also be used as turn-off criterion, forexample in that a limit is determined for the increase speed of thetissue impedance and the reaching thereof is monitored.

In any event, the energy applied so far is stored at the end of theoperating phase I. The progress of further controlling operating phaseII is derived from this energy value. In particular, the duration ofoperating phase II can be defined according to the energy value fromoperating phase I. The turn-off criterion, that is, the end of asubsequent operating phase III, can also be defined by means of theenergy value determined in the first operating phase. The controlparameters, that is, the duration of operating phase II and the turn-offcriterion, that is, the end of operating phase III, are thus functionsof the energy measured in operating phase I. Preferably, the transitionfrom operating phase I to operating phase II takes place continuously,that is, without abrupt change of the current supplied to the biologicaltissue and/or without abrupt change of the voltage applied to the tissueand/or without abrupt change of the power output to the tissue.

In operating phase II, the control unit preferably operates the sourcein an impedance-controlled manner as reference value of the impedanceincrease. A value of above 100 Ohm per second is recommended for thetissue impedance. The specific slow increase of the impedance causes astabilization of the evaporation of tissue fluid. The vapor formationtakes place evenly and in a spatially distributed manner. The desiredchronological course of the impedance can have a constant increase oralso a variable increase. Preferably, the control unit defines thechronological length of operating phase II as a function of the energydetermined in the first operating phase. The second operating phase isconcluded, when the time t₂ has elapsed. The third operating phase IIIfollows (optionally). During the latter, a constant voltage ispreferably applied to the biological tissue.

The end of the third operating phase III can be defined in that theminimum treatment time has elapsed and an energy E_(tot) has beenreached. The energy E_(tot) can be defined as a function of the energyE₁ determined in the first operating phase. The minimum treatment timet_(min) can also be determined by the energy E₁. In the alternative, theoperating phase III can be concluded, when the maximum treatment timehas elapsed. The latter, in turn, can be defined as a function of theminimum treatment time and thus also as a function of the energy E₁determined in the first operating phase. Further turn-off criteria,which in each case are a function of the energy E₁, can be defined.

During the course of the treatment, it may happen that treatmentparameters change. For example, inadvertent temporary loosening of theelectrodes from the biological tissue (opening of fusion clamps),seeping tissue fluid, such as blood or rinsing fluid, can influence theprocess. It may thus become necessary that a larger quantity of energyand longer application time becomes necessary, than was originallyderived from the energy E₁. To attain proper fusion in such cases, thecurrent power can be monitored during the second (and/or third)operating phase. Provided that the power within a monitoring timeinterval leaves a predetermined window of minimum power P_(min) andmaximum power P_(max) for a non-negligibly short period of time, theapplication time, that is the times t₂ and t₃, as well as calculatingparameters t_(min) and/or t_(max) can be lengthened accordingly.

Further details of embodiments of the invention follow, from the drawingand/or from the following description of an illustrative example:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the device according to the invention in schematicillustration.

FIG. 2 shows a control unit for the device according to FIG. 1, in asectional schematized block diagram and

FIG. 3 shows time diagrams for explaining the function of the controlunit.

DETAILED DESCRIPTION

FIG. 1 illustrates a device 10 for coagulating biological tissue 11,which can be a hollow vessel or also any other biological tissue, forexample. In the following example, a blood vessel is illustrated astissue 11, which is to be closed by means of coagulation, that is, afusion of the walls of the vessel, which are located opposite oneanother, is to be carried out. Two electrodes 12, 13, which can seizethe tissue 11 between one another and which can also stress itmechanically, for example by means of compression, serve this purpose.The mechanical structure of the corresponding instrument is notillustrated in detail in FIG. 1. For example, the electrodes 12, 13 canbe the branches of a bipolar fusion instrument.

The electrodes 12, 13 are connected to a feeding device 15 via a line14. For this purpose, the line 14 encompasses two leads 16, 17, forexample, to which the device 15 supplies or can supply high-frequencycurrent.

For this purpose, the device 15 encompasses a source 18, for example inthe form of a controllable HF generator 19. The latter can be suppliedwith operating voltage via a power supply 20 and a power connector 21via a mains power supply.

The HF generator 19 and/or the power supply 20 are embodied so as to becontrollable. At their corresponding controls inputs, a control unit 22,which controls or regulates in particular the output of electric powerthrough the HF generator 19, is connected, as is illustrated by means ofarrows. For this purpose, the control unit 22 includes a monitoring unit23, which determines the electric variables of the electric energy,which is supplied to the electrodes 12, 13. In particular, themonitoring unit 23 is equipped to determine and integrate the electricpower supplied to the electrodes 12, 13 at least temporarily, so as toestablish the energy, which is supplied during a time interval. Themonitoring unit 23 can encompass a voltage block 24 for monitoring thevoltage applied at the clamps 12, 13. In addition, the monitoring unit23 can encompass a current block 25 for establishing the size of thecurrent, which is supplied to the electrodes 12, 13. The control unit 22can furthermore encompass a module 26 for defining the conclusion of afirst operating phase I, wherein the module receives at least one outputsignal from the voltage block 24 or from the current block 25 or asignal derived from the output signals thereof for recognizing the endof the operating phase.

In FIG. 2, the control unit 22 is illustrated in a schematicallysimplified manner and only in excerpts. The current block 25 determinesthe current I_(ACT), which flows through the tissue 11. The actualvoltage U_(ACT), which is applied to the tissue 11, is determined bymeans of the voltage block 24. The power P_(ACT), which is actuallysupplied to the tissue 11, is calculated from both variables, at leasttemporarily. The power P_(ACT) can be the determined active power oralso the apparent power, which is supplied to the electrodes 12, 13. Acorresponding block serves the purpose of calculating the power P_(ACT)or for determining it otherwise.

The control unit 22 can furthermore encompass a current default block28, which provides a current I_(REF) as a function of time and/orsituation. Likewise, provision can be made for a voltage default block29, so as to provide a desired voltage U_(REF). The current defaultblock 28 and the voltage default block 29 can be controlled by animpedance block 30, which defines a desired relationship between thevoltage U_(REF) and the current I_(REF) as a function of time orsituation, for example so as to define a desired tissue resistance R_(G)or a desired chronological course thereof.

The reference-actual deviations for the current I_(ACT) and the voltageU_(ACT) are in each case formed in corresponding differential formingblocks 31, 32 and are supplied to a processing module 33. The lattercontrols the generator 19.

The processing module 33 furthermore includes the module 26 forrecognizing various operating phases. This module 26 can obtain at leastthe actual current I_(ACT) and/or the actual voltage U_(ACT) or a value,which is derived from these variables, as input variable (vianon-illustrated signal paths).

An energy block 34 for determining the energy supplied to the tissue 11is connected to the block 27 for establishing the power. Said energyblock integrates the measured power P_(ACT) for a period of time, whichis provided by the processing module 33, and supplies the integral tothe processing block 33.

It is pointed out that the blocks 27 to 32 as well as 34 can also bepart of the processing module 33.

The further design of the device 15 and in particular of its controlunit 22 follows from the following description of the time behaviorthereof:

It is assumed that living, non-denaturized tissue 11, is initiallyseized between the electrodes 12, 13. At its activation input 35, thedevice 15 now receives the signal for coagulation and, if applicable,for fusion of the biological tissue 11. This corresponds to the startingpoint or activation onset t₀, respectively, which is noted in FIG. 3.Operating phase I initially starts with a partial phase Ia. In thelatter, the current I_(ACT) is brought to a desired current value of 4A, for example, in a controlled manner. The current can thereby bebrought from an initial value, such as 1 A, for example, to thereference value of 4 A, for example, within a period of time t_(1a).This can take place in a linear ramp: the time for this can be between200 ms and 2 s. Preferably, the effective value of the current is usedas measuring variable. The tissue resistance R_(G) decreases from aninitial value to a minimum value of between 2 Ohm and 40 Ohm, forexample, during this phase or also completely or partially in a lateroperating phase Ib. Due to the increase of the current, the voltageU_(ACT) increases during the time period t_(1a). During this time, thecurrent I_(ACT) is preferably increased in the form of a ramp. Forexample, the peak voltage between the electrodes 12, 13 can be measuredas measuring value for the voltage U_(ACT). In operating phase I, thecurrent I_(ACT) is then held constant at the value i_(1b) during afurther partial phase Ib. The control unit 22 thereby operates ascurrent regulating circuit for keeping the value i_(1b) constant.

During the first partial phase Ia or during the second partial phase Ib,the tissue resistance R_(G) passes through a minimum, so as to thenincrease again. If the tissue resistance minimum is already reached inthe first partial phase Ia, the partial phase Ib can be skipped and adirect transition into operating phase II can be made. The power limitof the generator 19 might possibly be reached thereby, so that thecurrent regulating circuit is no longer able to bring the currentI_(ACT) into conformity with the desired current I_(REF). Towards theend of operating phase I, the current thus decreases. Depending on theembodiment, this decrease of the current i_(1b) or also the currentdifferential value (I_(REF)−I_(ACT)), which is formed by thedifferential forming block 31, can be used as indicator for theconclusion of operating phase I. It is also possible for the controlunit 22 to establish the tissue impedance R_(G) as quotient from U_(ACT)and I_(ACT) and to determine the conclusion of operating phase I, if thetissue resistance exceeds a given threshold. In the alternative, theincrease speed for the tissue resistance R_(G) can also be monitored.According to this, the control unit 22 can use the following criteria torecognize operating phase I, either cumulatively or as alternatives:

-   -   detecting the pass-through of the minimum of the tissue        impedance or of the tissue resistance dR/dt=0)    -   falling below a threshold value of the current I_(ACT), for        example 0.5*i_(1b)    -   exceeding a threshold value of the tissue impedance, for example        80 Ohm    -   exceeding a threshold value of the increase speed of the tissue        impedance (dR/dt).

During the entire operating phase I, the energy block 34 integrates thepower established by the block 27 and supplies the established value ofthe energy E₁ to the processing module 33 at the conclusion of operatingphase I. The onset and the conclusion of operating phase I are marked bymeans of the points in time t₀ and t₁. The point in time t₁ isdetermined by the processing module 33 according to one of theabove-mentioned criteria.

Operating phase II starts with the conclusion of operating phase I.Operating phase II preferably starts with the same current I_(ACT), withwhich operating phase I concluded. In addition, it preferably beginswith the same voltage U_(ACT), with which the first operating phase Iconcluded. Operating criteria are now defined for operating phase II bymeans of the applied energy E₁, which was established in operating phaseI. In operating phase II, the generator 19 is preferably operated in animpedance-regulated manner, that is, the control unit 22 forms aregulator for the tissue impedance. A desired chronological impedanceincrease A is defined for the tissue impedance. In FIG. 3, the impedanceincrease A is illustrated as R_(Gref) as desired dashed line over thecourse of time. The actual impedance increase R_(Gact) can deviateslightly from this. This depends on the control quality of the impedanceregulator, which is now formed by the control unit 22. At the same time,the current I_(ACT) decreases during operating phase II, that is, duringthe period of time t₂, while the voltage U_(ACT) increases. The voltageU_(act) has an upper limit, e.g. 150V (peak value), so that it isavoided that sparks appear and that a cutting effect would thus becaused.

The impedance increase A can be between 50 and 200, preferably 100 Ohmper second. The specific slow increase of the impedance causes astabilization of the evaporation of the tissue fluid.

Operating phase II is concluded, when the period t₂ has elapsed. Theperiod t₂ can be established from the energy E₁ as follows:t ₂=2/3(t _(max) −t ₁).

The time t_(max) is thereby the maximum treatment period. The maximumtreatment period t_(max) can be calculated from the minimum treatmentperiod, in that a constant defined summand is added, for example:t _(max) =t _(min)+1.8 s

The minimum treatment period t_(min) can be determined, for example,from the following relationship from the energy E₁:t _(min)=min{5.4 s; (−38.25 μs*E ₁ ² /J ²+18 ms*E ₁ /J+270 ms)}.

According to this, t_(min) is a defined value of 5.4 s, for example, orwhich results from calculating the round bracket, depending on whichvalue is less.

With the conclusion of operating phase II, operating phase III begins.In the latter, the voltage U_(ACT) is constantly regulated to the valueU₃ for a period of time t₃. The control unit 22 operates as a voltageregulator circuit herein.

During operating phases II and III, which correspond to phase II of thetissue coagulation, the power is integrated further. When this valuereaches the total maximum value E_(tot), the treatment is concluded. Thetotal maximum value E_(tot) can be established according to variousempirically obtained formulas as a function of the energy E₁, forexample as follows:E _(tot)=45J+2.75*E ₁.

In the alternative, the reaching of the maximum period t₃ of operatingphase III can be recognized. This period t₃ can be calculated, forexample according to:t ₃=1/3*(t _(max) −t ₁).

To avoid improper treatments caused by unforeseen changes of thetreatment parameters, for example by accidentally opening the fusionclamps, it can additionally be monitored, whether the actual powerleaves a performance window from P_(min) and P_(max) within a monitoringtime interval, for example during operating phase II and/or III.

A device 10 for tissue coagulation, in particular fusion, encompasses anelectric source 18, which is connected or can be connected to electrodes12, 13 for influencing biological tissue 11 with current. A control unit22 controls the source 18 during phases I and II of the tissue fusion.These phases I and II correspond to operating phases I, II and III ofthe device 10. During operating phase I, a monitoring device 23determines the energy E₁, which is applied into the tissue 11. Thecontrol unit 22 controls the source 18 in the subsequent operatingphases II and III by means of the determined energy E₁. Such a deviceturns out to be particularly reliable and to be robust in use.

LIST OF REFERENCE NUMERALS

-   10 device-   11 biological tissue-   12, 13 electrodes-   14 line-   15 device-   16, 17 leads-   18 source-   19 HF generator-   20 power supply-   21 power connector-   22 control unit-   23 monitoring unit-   24 voltage block-   25 current block-   26 module for recognizing operating phases-   U_(ACT) voltage (e.g. peak value)-   I_(ACT) current (e.g. effective value)-   P_(ACT) power-   27 block for establishing power-   28 current default block-   I_(ACT) desired current-   29 voltage default block-   U_(REF) desired voltage-   30 impedance block-   R_(G) tissue resistance-   31, 32 differential forming blocks-   33 processing module-   34 energy block-   35 activation input-   t₀ activation onset-   I first operating phase-   Ia partial phase-   t_(1a) period of the first partial phase-   Ib partial phase-   i_(1a) value of the current I_(ACT) in the partial phase Ia-   i_(1b) value of the current I_(ACT) in the partial phase Ib-   t₁ period of operating phase I-   E₁ energy input into the tissue 11 in phase I-   A impedance increase-   R_(Gref) desired impedance course-   R_(Gact) actual impedance course-   t₂ period of operating phase II-   t_(max) maximum period of treatment-   t_(min) minimum period of treatment-   E_(tot) total maximum value of the energy-   t₃ period of operating phase III-   t_(tot) total period of treatment-   R_(Gmax) threshold value for tissue resistance in operating phase I-   M minimum of the tissue resistance in operating phase I-   U₃ voltage in operating phase III-   P_(max), P_(min) define performance windows for the power P of the    source 18 in operating phases II and/or III

What is claimed is:
 1. A device (10) for tissue coagulation, the devicecomprising: an electric source (18), configured to be connected toelectrodes (12, 13) for influencing biological tissue (11) with current,a monitoring unit (23), which is connected to the electric source (18),configured to determine one or both of a current (I_(ACT)) output by theelectric source (18) and a voltage (U_(ACT)) is output by the electricsource (18), a control unit (22), which includes the monitoring unit(23) and which is connected to the electric source (18) in a controllingmanner, the control unit (22) configured to: establish an energy (E1)the electric source (18) outputs to the electrodes (12, 13) in a firstoperating phase (I), and control the electric source (18) as a functionof the energy (E1), which is determined in the first operating phase(I), in a subsequent second operating phase (II), and to turn off theelectric source (18) after reaching a total energy output to theelectrodes (12, 13), wherein the total energy is determined as afunction of the energy (E1).
 2. The device according to claim 1, whereinthe control unit (22) is interconnected with the electric source (18) inthe first operating phase (I) as a current regulating circuit.
 3. Thedevice according to claim 1 wherein at an onset of the first operatingphase (I), the control unit (22) is configured to define achronologically increasing current (i1 a).
 4. The device according toclaim 1 wherein during at least a section (Ib) of the first operatingphase (I), the control unit (22) is configured to define a constantcurrent (i1 b).
 5. The device according to claim 1 wherein the controlunit (22) comprises a module (26) configured to determine a conclusionof the first operating phase (I) using at least one of: a relationshipbetween voltage and current at the electric source (18) increases beyonda threshold value (R_(Gmax)), the relationship between voltage andcurrent at the electric source (18) passes through a minimum (M), anincreased speed of change in the relationship between voltage andcurrent at the electric source (18) exceeds a threshold value, thevoltage (U_(ACT)) at the electric source (18) exceeds a threshold value,the current (I_(ACT)) falls below a threshold value.
 6. The deviceaccording to claim 1 wherein at an onset of the second operating phase(II), the control unit (22) is equipped to adjust at least one variableof: current (I_(ACT)) from the electric source (18), voltage (U_(ACT))at the electric source (18), output power (P_(ACT)) of the electricsource (18) to a same value the variable had at a conclusion of thefirst operating phase (I).
 7. The device according to claim 1 wherein inthe second operating phase (II), the control unit (22) is configured todefine a course of time for changing of a relationship between thevoltage (U_(ACT)) at the electric source (18) and the current (I_(ACT))supplied by said source.
 8. The device according to claim 7, wherein thecourse of time encompasses a constant impedance increase (A).
 9. Thedevice according to claim 1 wherein the control unit (22) is configuredto define a chronological length (t2) of the second operating phase(II).
 10. The device according to claim 1 wherein the control unit (22)is configured to define a chronological length (t2) of the secondoperating phase (II) as a function of the energy (E1), which isdetermined in the first operating phase (I).
 11. The device according toclaim 1 wherein directly following the second operating phase (II), thecontrol unit (22) is configured to merge into a third operating phase(III).
 12. The device according to claim 11, wherein in the thirdoperating phase (III), the control unit (22) is configured to define andadjust a constant voltage (U3).
 13. The device according to claim 11,wherein in the third operating phase (III), the control unit (22) isconfigured to define a voltage (U3) of the electric source (18) to avalue determined by the monitoring unit (23) at a conclusion of thesecond operating phase (II).
 14. The device according to claim 11,wherein the control unit (22) is configured to conclude the thirdoperating phase (III), if: a minimum treatment time (t_(min)) and agiven total energy (E_(tot)) have been reached or a maximum treatmenttime (t_(max)) has elapsed or a maximum energy (E_(max)) has beenapplied.
 15. The device according to claim 11, wherein the control unit(22) is equipped to monitor a power output by the electric source (18)in the operating phase (II), so as to extend a time period (t2) for thesecond or third operating phase (II, III), provided that the power hasleft a performance window, which is defined between a maximum power(P_(max)) and a minimum power (P_(min)).